Applicant claims priority under 35 U.S.C. xc2xa7119 of German Application No. 101 38 056.9 filed Aug. 3, 2001.
1. Field of the Invention
The present invention relates to a turbo-machine. The turbo-machine includes a housing; a rotor shaft; a rotor of a centripetally flow-through turbine expansion stage that is mounted on one end of the rotor shaft; and a rotor of a radially flow-through compressor stage that is mounted on the other end of the rotor shaft.
2. The Prior Art
Turbo-machines with the structure described above, which are referred to also as turbo-expanders, are preferably operated at high pressure levels in industrial applications. In such applications, the outlet pressure of the turbine, which is directed in the axial direction of the rotor shaft, frequently deviates to a high degree from the pressure on the compressor inlet, which is axially directed as well. High axial forces of thrust are exerted on the rotor shaft operating at a high number of revolutions because of the in pressure between the compressor stage and the expansion stage of the turbine. The axial thrust can be completely absorbed by an adequately dimensioned axial bearing; however, axial bearings rated for absorbing high axial operating thrusts generate considerable loss capacities. The loss capacity of the axial bearings rated for the least favorable operating conditions may amount to from 5% to 10% of the expansion capacity of the turbine. The driving capacity available in the compressor stage is diminished accordingly. This reduces the overall degree of efficiency of a plant, for example a refrigeration plant in which the turbo-machine is used.
The use of inclined high-precision ball bearings, which produce only low losses even at very high numbers of revolution, seems to be advantageous. Such inclined high-precision ball bearings can be operated in specially designed types of turbines with a product of the number and diameter of up to 2.5xc3x97106 mm/min. This value is adequate for operating, for example a refrigeration turbo-expander with an expansion pressure ratio of pin/pout=10 in the expansion stage of the turbine. However, high-precision roller bearings will reach an acceptably long useful life only if the roller bearings are subjected to low forces, in particular low axial forces in a continuous operation. The permissible values are not adequate in the normal case for absorbing the axial thrusts and shocks occurring in the course of operation of a turbo-expander.
The invention addresses the problem of providing in a turbo-expander compensation for axial thrust that can be controlled automatically and will also automatically adapt itself to different operating conditions of the turbo-expander.
The problem is solved in accordance with the invention with a turbo-machine including a housing; a rotor shaft; a rotor of a centripetally flow-through turbine expansion stage, the rotor being overhung on one end of the shaft of the rotor; a rotor of a radially flow-through compressor stage, the rotor being overhung on the other end of the shaft of the rotor; a measuring system for directly detecting any axial thrust of the rotor shaft supported in roller bearings; and at least two pressure control valves controlled by a controller in control lines.
In accordance with the invention, a surface located on the back side of one of the rotors and sealed against the housing, can be acted upon by the pressures prevailing on the inlet and outlet of the stage. Depending on the measured axial thrust, the controller actuates either the pressure control valve in the control line under the higher pressure or the pressure check valve that is under the lower pressure. Any axial pressure outside of a preset tolerance range is thereby eliminated by raising or reducing the pressure acting on the surface of the rotor piston.
The rear side of the compressor rotor may be provided with the surface of the rotor piston. The pressure prevailing in the compressor stage and the pressure on the outlet of the compressor stage can be admitted to the surface via the control lines. The controller will actuate according to the measured axial thrust either the pressure control valve located in the control line through which the compressor inlet pressure is admitted, or the pressure control valve installed in the control line that is under the compressor outlet pressure. However, also within the scope of the invention is an embodiment in which the surface of the rotor piston is provided on the rear side of the rotor of the expansion stage of the turbine.
A permissible axial thrust is fixed for the roller bearing of the rotor shaft. During the operation of the turbo-machine, the controller compares the axial thrust values recorded by the measuring system with the preset range of the nominal values. If the measured axial thrust is outside of the range of the nominal values, one of the two pressure control valves is controlled to correct the axial thrust. The pressure acting on the surface of the rotor piston can be controlled by the pressure control valves in a highly sensitive manner. The pressure acting on the surface of the rotor piston can be raised or reduced depending on the direction in which the axial thrust needs to be corrected.
According to a further development in accordance with the invention, additional control lines may be provided which each comprise a valve. Also provided is an additional surface of the rotor piston, which is sealed against the housing and located on the rear side of the other rotor. The additional piston surface can be acted upon by the pressure prevailing on the inlet of this stage and the pressure on the outlet of this stage by way of the additional control lines.
The valves of the additional control lines can take the form of pressure control valves that are controlled by the controller as well. In this way, the controller controls the pressure control valves installed in the control lines as well as in the additional control lines. The valves are controlled so that the changes in pressure occurring on the rotor piston surfaces of the compressor rotor and the rotor of the turbine complement one another so that the force of compensation for compensating the axial thrust is intensified. To that extent, the pressure control valves are wired, for example so that an increase in the pressure on the rotor piston surface of the compressor stage is combined with a reduction in the pressure on the rotor piston surface of the turbine rotor. Any reduction in the pressure acting on the rotor piston surface of the compressor rotor is accordingly combined with an increase in the pressure acting on the rotor piston surface of the turbine rotor. In this embodiment, very high forces of compensation may be raised and controlled precisely. In this way, the axial thrust of the rotor shaft may be maintained within a narrow range of the nominal value that is permissible for high-precision roller bearings.
It is within the scope of the invention as well that the valves of the additional control lines are rapidly opening switching valves, for example magnetic valves. Rapidly opening switching valves are useful to rapidly correct abrupt changes occurring in the axial thrust. The lower or higher pressure is consequently rapidly admitted in full strength to the additional surface of the rotor piston.
According to a preferred embodiment of the invention, the measuring system includes a ring-shaped force absorber that is arranged between an outer bearing ring of a roller bearing and the corresponding stop surface of the housing. The ring is located on the side of the housing and stressed by the axial thrust of the rotor shaft. The force absorber may include extensometer (or expansion measuring) strips and thus measures independently of time unchanging axial thrusts as well. For example, an initial axial tensioning force of the bearing in the idle state may be measured without pressure being admitted to the turbo-machine.